Cardiovascular disease (CVD) is prevalent in the world and is often associated with other disease states such as diabetes and obesity. Many population studies have attempted to identify the risk factors for CVD; of these, high plasma levels of low density lipoprotein cholesterol (LDL-C), high plasma levels of triglycerides (>200 mg/dl), and low levels of high density lipoprotein cholesterol (HDL-C) are considered to be among the most important. Currently, there are few therapies targeting low HDL-C and triglycerides.
The peroxisome proliferator-activated receptors (PPARs) are metabolic sensors regulating the expression of genes involved in glucose and lipid homeostasis. Agonists of the PPARα subtype, such as LOPID® (gemfibrozil) and TRICOR® (fenofibrate), and agonists of the PPARγ subtype, such as AVANDIA® (rosiglitazone maleate), are used for the treatment of dyslipidemia and diabetes, respectively. Another member of this nuclear receptor family, the peroxisome proliferator-activated receptor delta (PPAR delta or PPARδ) is also a necessary transcription factor reported to be involved in regulating genes involved in lipid metabolism and energy expenditure. PPAR delta has been shown to act as a “gateway” receptor modulating the expression of the other PPARs (Shi et al., 2002, Proc Natl. Acad. Sci USA, 99(5): 2613–2618). Each receptor subtype has a distinct tissue distribution: 1) PPARα shows the highest expression in liver, 2) PPARγ appears primarily in adipose tissue, and 3) PPARδ has the widest distribution—ubiquitously in adult rat (Braissant et al., 1996, Endocrinology 137(1): 354–366) and in all the human tissues tested to date, including liver, kidney, abdominal adipose and skeletal muscle (Auboeuf et al., 1997, Diabetes 46(8):1319–1327).
Recently, potent ligands for PPARδ have been published, providing a better understanding of its function in lipid metabolism. The main effect of these compounds in db/db mice (Leibowitz et al., 2000, FEBS Lett. 473(3):333–336) and obese rhesus monkeys (Oliver et al., 2001, Proc. Natl. Acad. Sci. USA 98(9):5306–5311) was an increase in high density lipoprotein cholesterol (HDL-C) and a decrease in triglycerides, with little effect on glucose (although insulin levels were decreased in monkeys). HDL-C removes cholesterol from peripheral cells through a process called reverse cholesterol transport. The first and rate-limiting step, a transfer of cellular cholesterol and phospholipids to the apolipoprotein A-I component of HDL, is mediated by the ATP binding cassette transporter A1 (ABCA1) (Lawn et al., 1999, J. Clin. Investigation 104(8): R25–R31). PPARδ activation has been shown to increase HDL-C level through transcriptional regulation of ABCA1 (Oliver et al., 2001, Proc. Natl. Acad. Sci. USA 98(9): 5306–5311). Through induction of ABCA1 mRNA expression in macrophages, PPARδ agonists may increase HDL-C levels in patients and remove excess cholesterol from lipid-laden macrophages, thereby inhibiting the development of atherosclerotic lesions. Existing therapy for hypercholesterolemia includes the statin drugs, which decrease LDL-C but show little effect on HDL-C, and the fibrates, the PPARα agonists that have low potency and induce only modest HDL-C elevation. In addition, like the fibrates, PPARδ agonists may also reduce triglycerides, an additional risk factor for cardiovascular disease and diabetes. Elevated free fatty acid level has been shown to contribute to insulin resistance and progression of diabetes (Boden, G. PROCEEDINGS OF THE ASSOCIATION OF AMERICAN PHYSICIANS (1999 May-June), 111(3), 241–8).
Examples of known PPAR delta agonists variously useful for hyperlipidemia, diabetes, or atherosclerosis include L-165041 (Leibowitz et al., 2000) and GW501516 (Oliver et al., Proceedings of the National Academy of Sciences of the United States of America (2001), 98(9), 5306–5311). Treatment of differentiated THP-1 monocytes with GW501516 induced ABCA1 mRNA expression and enhanced cholesterol efflux from these cells.